In recent years, it is important to cope with environmental problems in various fields. Technical development on energy saving and reduction of a discharge amount of carbon dioxide is thus being promoted. For example, it is an important subject to improve the fuel efficiency for vehicles, and for attaining the subject, technological development of lubricating oils and sliding materials is carried out.
As regards development of lubricating oil compositions, various base oils and additives have so far been developed for the purpose of improving various performances. As performances required to engine oils, there may be mentioned, for example, appropriate viscosity characteristics, oxidation stability, detergent dispersibility, abrasion resistance and antifoaming property. These performances are attempted to be improved by combination of various base oils and additives. In particular, zinc dialkyldithiophosphate (ZnDTP) is excellent as an abrasion resistant additive and, therefore, is often used as an additive for engine oils.
As regards development of sliding materials, on the other hand, materials having a hard film such as a TiN film or a CrN film which contributes to an improvement in an abrasion resistance are known as materials for use in parts which are exposed to severe frictional and abrasive environments (for example, a sliding part of an engine). Further, it is known that a friction coefficient can be reduced in the air in the absence of a lubricating oil by utilizing a diamond-like carbon (DLC) film. Thus, a material having a DLC film (hereinafter referred to as a DLC material) is expected as a low-friction sliding material.
However, the friction reducing effect of a DLC material is occasionally small in the presence of a lubricating oil composition, and, in this case, a fuel consumption saving effect is less liable to be obtained. Accordingly, development of a lubricating oil composition for low-friction sliding materials such as DLC materials or the like has heretofore been carried out.
For example, a lubricating oil composition for a low-friction sliding member which contains an ether-based ashless friction reducing agent is disclosed in PTL 1. Disclosed in PTL 2 and 3 are techniques in which lubricating oil compositions containing fatty acid ester-based ashless friction controlling agents and aliphatic amine-based ashless friction controlling agents are used for a sliding face between a DLC member and an iron base member and a sliding face between a DLC member and an aluminum alloy member. Disclosed in PTL 4 is a technique in which a low-friction agent composition containing an oxygen-containing organic compound and an aliphatic amine-based compound is used in a low-friction sliding mechanism having a DLC coating sliding member.
As in the above, lubricating oil compositions for low-friction sliding materials have been developed; however, even though these techniques are applied, the friction coefficient may increase when ZnDTP is incorporated for further improving abrasion resistance, and there is recognized a phenomenon that could not attain friction reduction.
Accordingly, even such a lubricating oil composition that contains, for example, ZnDTP for maintaining and improving various performances required for lubricating oil compositions, there is still required a lubricating oil composition that shows an extremely low friction coefficient when used as a lubricating oil for low-friction sliding materials.
There is also desired a sliding mechanism excellent in low-friction performance, which uses a lubricating oil composition capable of exhibiting excellent low-friction performance while maintaining various characteristics as such lubricating oil compositions, as combined with a sliding member that uses the above-mentioned DLC film on the sliding surface thereof.